Diesel engines and other lean-burn engines or power plants are operated at higher than stoichiometric air to fuel mass ratios for improved fuel economy. Such lean-burning engines produce a hot exhaust with a relatively high content of oxygen and nitrogen oxides (NOx). The temperature of the exhaust from a warmed-up diesel engine is typically in the range of 200° C. to 400° C. and has a representative composition, by volume, of about 10-17% oxygen, 3% carbon dioxide, 0.1% carbon monoxide, 180 ppm hydrocarbons, 235 ppm NOx and the balance nitrogen and water. These NOx gases, typically comprising nitric oxide (NO) and nitrogen dioxide (NO2), are difficult to reduce to nitrogen (N2) because of the high oxygen (O2) content in the hot exhaust stream.
Traditional three-way catalysts used with stoichiometric gasoline engines are not very effective for treating lean-burn exhaust. Researchers have attempted to find durable catalysts that can selectively reduce NOx using engine-out hydrocarbons (HC-SCR) despite the competing combustion reaction with the oxygen content of the exhaust. Such catalyst developments have not been successful because of poor activity, narrow operating temperature window and insufficient durability of candidate catalyst materials.
One of the hydrocarbon-utilizing technologies now being considered for the lean-burn gasoline or diesel engine NOx emission control is to reduce NOx using selected fuel-component hydrocarbons added to the exhaust stream. In one such approach, ozone is also added to the exhaust to promote the oxidation of NO to NO2 and the exhaust with its added hydrocarbon and ozone is passed over a selective reduction catalyst comprising, for example, a dual bed of BaY zeolite (upstream bed) and CuY zeolite (downstream bed). In the oxidizing atmosphere of the exhaust stream some of the hydrocarbon species are partially oxidized to aldehydes or alcohols which promote the reduction of NO2 when the stream passes in contact with the selective reduction catalyst.
The present specification describes a further and related improvement for more effective partial oxidation of added hydrocarbon for the selective catalytic reduction of NO and NO2 in exhaust streams from lean-burn engines and power plants.